Liquid photopolymers are used in the prior art of printed wiring board manufacture, as evidenced in my U.S. Pat. No. 4,260,675 in which a process is disclosed for preparing a glass plate phototool which carries as opaque pillars or opaque stand-offs the image of the solder mask pattern. Thus, a solder mask insulation layer is prepared by coating the board overall with a thin layer of liquid photopolymer, e.g., by screen printing using a mesh fabric without a stencil, and placing the glass plate phototool onto the board in register so that the opaque pillars register correctly with the land areas and are firmly embedded in the liquid photopolymer, so as to shield the underlying photopolymer from radiation from a UV light source. Thus, the surrounding photopolymer areas are polymerized, and transformed to a well adhered solid leaving protected area under the pillars unpolymerized and remaining in the liquid state. A solvent washout step removes unpolymerized photoresist from the printed wiring board, leaving the land areas free of solder mask and capable of receiving a coating of molten solder.
There are three features of the method of U.S. Pat. No. 4,260,675 which this disclosure seeks to improve; that is to reduce the waste of liquid photopolymer experienced when the entire board surface is coated; and to prevent the entry of liquid photoresist into drilled holes, for a considerable amount of time is required to clear the holes of resist during the washout step. The third improvement sought is the prevention of photopolymer adhering to the pillars, for after repeated exposure cycles there is a tendency for the photopolymer to bond the pillars to the board surface.
In U.S. Pat. No. 4,436,806--Rendulic et al. describes a method for producing relief images on printed wiring boards wherein a coated board is exposed through a phototransparency held above and off-contact the liquid photopolymer and exposed by collimated light directed through the transparency. Thus, while there is no contact between the transparency and the liquid photopolymer there is an inherent disadvantage in this method, for collimated light is required in order to prevent light undercutting the transparency opaque areas which would introduce intolerable loss of image fidelity. Collimated light is expensive to produce, especially the power levels required to polymerize a layer of photoresist of the order of 0.004 inches thick. Additional undesirable features are the waste of photopolymer due to coating the entire board; and the seepage of liquid resist into the holes, which requires a longer washout cycle.
A third example of prior art liquid photopolymer imaging is described in my U.S. Pat. No. 4,424,089 in which a flexible phototransparency is coated overall with a layer of liquid photopolymer then laminated onto the printed wiring board in correct register. The photopolymer is exposed to a source of uncollimated actinic light to harden light-struck photopolymer and adhere the images to the printed wiring substrate.
As with the aforedescribed current art processes, the process of U.S. Pat. No. 4,424,089 is also wasteful of photopolymer, and liquid photopolymer is forced into drilled holes, and the washout time is extended thereby.
To place in perspective the waste of time and material resources inherent in the aforedescribed processes the following waste is experienced. On a typical computer grade printed wiring board of 3 square feet area, approximately 18 grams of liquid plating photoresist is applied and approximately 9 grams are washed out in the development step, and this is wasteful.
Further, the quantity of solvent required to dissolve and remove 9 grams of photoresist is of the order of 10 times the quantity of photoresist dissolved.
Time resources for development or washout are increased by a factor of 6 when liquid photopolymer plugs drilled holes, when compared to the development time when the holes are not plugged. Thus, a one minute washout time is reduced to 10 seconds. In the current art, liquid photoresists are applied to printed wiring boards in the proper patterns by screen printing, and this conserves photoresist and does not force photopolymer into drilled holes. However, the screen printed images inherently have poor resolution, owing to indistinct boundaries and slight smears. Thus, the ability to resolve fine lines and spaces is limited to 0.010 inches in a production environment. A second advantage of screen printing photoresist images is the speed; the production rate is of the order of 5 times that of photopatterning the images.
Among the objectives of this disclosure are to improve the state of the art by a simplified higher resolution process of screen printing, particularly by reducing production time, saving photoresist materials, and improving the quality of the images so as to extend the resolution of screen printed images below the 0.010 inch line and space limitation.